In drilling boreholes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters may be employed. The bit is secured to the lower end of a drill string that is rotated from the surface or by downhole motors or turbines. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drill string is rotated, thereby engaging and disintegrating the formation material to be removed. The roller cutters are provided with teeth or cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight from the drill string. The cuttings from the bottom and sidewalls of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow, rotating drill string and are carried in suspension in the drilling fluid to the surface.
It has been a conventional practice for several years to provide diamond or super-hard cutting elements or inserts in earth-boring bits known as PDC, or fixed cutter bits. The excellent hardness, wear, and heat dissipation characteristics of diamond and other super-hard materials are of particular benefit in fixed cutter or drag bits, in which the primary cutting mechanism is scraping. Diamond cutting elements in fixed cutter or drag bits commonly comprise a disk or table of natural or polycrystalline diamond integrally formed on a cemented tungsten carbide or similar hard metal substrate in the form of a stud or cylindrical body that is subsequently brazed or mechanically fit on a bit body.
Implementation of diamond cutting elements as primary cutting structure in earth-boring bits of the rolling cutter variety has been less common than with earth-boring bits of the fixed cutter variety. One reason is that the primary cutting elements of rolling cutter bits are subjected to more complex loadings, depending on their location on the cutters, making separation of the diamond tables from their substrates more likely. Moreover, because the loads encountered by the cutting elements of rolling cutter bits are typically much larger in magnitude than the loads sustained by the cutting elements of fixed cutter bits, stress concentrations caused by prior-art land and groove arrangements at the interface between the diamond and its substrate, such as shown by U.S. Pat. No. 5,379,854 to Dennis, can cause the diamond to crack or fracture.
One solution is found in U.S. Pat. Nos. 4,525,178; 4,504106; and 4,694,918 to Hall, which disclose cutting elements for a rolling cutter bit having the diamond and substrate formed integrally with a transition layer of a composite of diamond and carbide between the diamond layer and carbide layer. This transition layer is purported to reduce residual stresses between the diamond and carbide because the composite material reduces the differences in mechanical and thermal properties between the diamond and carbide materials. Another solution, disclosed in commonly assigned U.S. Pat. No. 5,119,714 to Scott, is to form a hard metal jacket around a diamond core. Unfortunately, these can be more difficult to manufacture than conventional flat PDC parts and are subject to costly and complex finishing operations.
A need exists, therefore, for diamond cutting elements or inserts for earth-boring bits of the rolling cutter variety that are sufficiently durable to withstand the rugged downhole environment and that are economical to manufacture.